It is possible that your HG leak is minor, which is why they couldn't really identify it. In which case, this might be all that you need to get rolling :

This might be the best first option for you.

If it fixes the problem, it will mean that there's no need for you to change the WP and thermostat and all the expense of that, although it is the intelligent thing to do. Perhaps you could postpone that till later when you feel like doing a full head job.

If this is the problem, you are fully justified in demanding a refund from the dealership, even if you fix it some time later. They should bloody well do a compression test on your car for that amount of water vapour shown, and for such a high price for a mere diagnostic.

And while you're there to collect the car, please double check to see if you can get them to show you how the stomp test works. And if it doesn't, they should fix it under warranty or recall or whatever.